This invention is particularly directed to temperature measurements made in the operation of solar energy systems wherein, as an example, temperature measurements are made of a solar collector, a rock storage unit, and a building temperature. Most such systems employ copper-constantan thermocouples, one being used to measure a discrete one of the system perameters. The low level output of a thermocouple must be amplified, and, in prior systems, the output of individual thermocouples was individually amplified, and this has required that the amplifier have a low input offset drift and that a fixed offset be removed with either a potentiometer or a resistor before the system is placed in operation. Each amplified temperature measurement is then referenced to a thermal standard which is either an oven or a diode voltage generator. Solar equations must then be solved using two or more temperature measurements summed in a separate amplifier.
An alternate system uses thermistors, each being connected in a bridge to drive a discrete amplifier and the solar equations solved by using two or more outputs from the separate amplifiers, which are then summed in another amplifier. Such a system requires an extremely accurate reference voltage plus an extremely accurate thermistor to make each measurement.
The two systems discussed have several disadvantages. First, both systems deal with absolute temperatures when actually the difference temperatures are ultimately needed. Further, a different temperature is accurately obtained only when the absolute temperature of each measurement is accurately made. This is not easily accomplished since accuracy is degraded by amplifier offsets and gain error as well as sensor errors. To get around these problems, high quality components must be used. The system using copper-constantan thermocouples requires very stable amplifiers, a very accurate bias supply, the removal of offset, and the use of precision gain resistors. Both systems require laboratory calibration and precision components that often lead to long-term stability problems. It is believed that such systems will require regular maintenance to set offsets and gain adjustments.